The Colorado leader Venancio Flores launched the Liberating Crusade in 1863, an insurrection aimed at toppling Bernardo Berro, who presided over a Colorado–Blanco coalition (fusionist) government.

Their leader, Brigadier General Venancio Flores, a caudillo and an early proponent of fusionism, found himself without sufficient military resources to mount a sustained revolt and resorted to asking for intervention by Argentina.

[11][12] Since 1862, the Blancos had made repeated overtures to Paraguay, governed by dictator Carlos Antonio López, in an attempt to forge an alliance[13][14][15] that might advance both their interests in the Platine region.

He believed Argentina was working towards the annexation of both Uruguay and Paraguay, with the goal of recreating the Viceroyalty of the Río de la Plata, the former Spanish colony that once encompassed the territories of all three nations.

Solano López had, as far back as 1855, expressed this concern, commenting to the Uruguayan Andrés Lamas that "the idea of reconstructing [the old viceroyalty] is in the soul of the Argentines; and as a result, it isn't just Paraguay that needs to stand guard: your country, the Oriental Republic [of Uruguay], needs to get along with my own in order to prepare for any eventualities.

"[18] In late 1863, Solano López was mobilizing his army and was in talks with Urquiza, the leader of the dissident Argentine Federalists, to convince him to join the proposed Paraguayan–Uruguayan alliance.

Its foreign policy included the covert underwriting of opposition parties in Uruguay and Argentina, preventing strong governments that might threaten Brazil's strategic position in the area.

In Uruguay, the bank run by Irineu Evangelista de Sousa (Baron and later Viscount of Mauá) became so heavily involved in commercial enterprises that the economy depended on this source of continued capital flow.

[23][24] Life along the frontier between Rio Grande do Sul and Uruguay was often chaotic, with hostilities erupting between partisans of various cattle barons, cattle-rustling and random killings.

[26] Among the Brazilian land barons were David Canabarro and Antônio de Sousa Neto, both allies of Flores and former separatist rebels during the Ragamuffin War that had ravaged Rio Grande do Sul from 1835 until 1845.

The ongoing civil war prevented elections; therefore Atanasio Aguirre, president of the Uruguayan senate and a member of the Amapolas (the radical wing of the Blanco Party) replaced Berro, on an interim basis.

[37] The main factor, as historian Jeffrey D. Needell summarized, was that the "Uruguayan president had been unwilling to resolve these, particularly because the Brazilians whose grievances were at issue were allies of Venancio Flores, a client of the Argentines, and a man who was seeking his overthrow.

"[38] A mutual enmity between Brazil and its Hispanic-American neighbors compounded the difficulties, the result of a long-standing distrust and rivalry between Spain and Portugal that had been carried over to their former American colonies.

The battle ended when the Villa del Salto ran aground near Paysandú, where its crew set it on fire to prevent it falling into Brazilian hands.

[61] Meanwhile, Aguirre had sent General Juan Sáa with 3,000 men and four cannons to relieve the besieged town, forcing the Brazilians and Colorados to briefly lift the siege while dealing with this new threat.

The Army of the South, stationed in Piraí Grande, was commanded by Field Marshal João Propício Mena Barreto (later Baron of São Gabriel) with two divisions.

[D] The army was poorly equipped for siege operations: it brought along no engineers (who could direct the construction of trenches); it was under-equipped, lacking even hatchets (necessary to cut fences, break through doors and scale walls); and its 12 cannons (a mix of La Hitte and Paixhans) were of small calibers ill-suited to attacking fortifications.

The Uruguayan gauchos "had combat experience but no training and were poorly armed save for the usual muskets, boleadoras, and facón knives", remarked historian Thomas L.

[66] "Fire arms he [the Uruguayan gaucho] rarely possessed", said Cunninghame Graham, "or if by chance he owned a pair of long brass-mounted pistols or a flintlock blunderbuss, they were in general out of order and unserviceable.

"[67] Field Marshal Barreto reached Paysandú on 29 December with two infantry brigades and one artillery regiment under Lieutenant Colonel Émile Louis Mallet (later Baron of Itapevi).

While the Army of the South crossed Uruguay heading toward Paysandú, Brazil's government sent José Maria da Silva Paranhos (later Viscount of Rio Branco) to replace Saraiva.

[81] Increasingly isolated, Aguirre hoped that the foreign powers could intervene, but when, on 11 January, he asked the diplomatic corps in Montevideo whether they would provide military assistance to him and his government, none responded positively.

[82][83] João Propício Mena Barreto sailed from Fray Bentos on 14 January with the Brazilian infantry, bound for a landing near the mouth of the Santa Lucía River near Montevideo.

Until then, there had been no formal declaration of war, and the Empire's military operations in Uruguay since August 1864 had been mere "reprisals"—the vague term used by Brazilian diplomacy since the ultimatum.

[87] In an attempt to divert the attention of Brazil from the siege of the capital, the Blanco government ordered the "Vanguard Army of the Republic of Uruguay", composed of 1,500 men under General Basilio Muñoz, to invade Brazilian soil.

[89] On 16 February, the Army of the South was further reinforced by 1,228 men from the 8th Battalion of Caçadores (Sharpshooters) arriving from the Brazilian province of Bahia, raising its numbers to 8,116.

French, Italian and Spanish troops landed in Montevideo at Villalba's request to dissuade the radical Blancos from attempting a coup to retake power.

Flores and Manuel Herrera y Obes (representing Villalba's government) signed a peace accord on 20 February at the Villa de la Unión.

[100][101] But public opinion quickly changed for the worse, when newspapers began running stories painting the accord of 20 February as harmful to Brazilian interests, for which the cabinet was blamed.

Not only had Paranhos managed to settle all Brazilian claims, but by avoiding the death of thousands, he gained a willing and grateful Uruguayan ally, not a dubious and resentful one—who provided Brazil an important base of operations during the war with Paraguay that followed.